Cation adsorption at permanently (montmorillonite) and variably (quartz) charged mineral surfaces: Mechanisms and forces from subatomic scale

Abstract

In this study, the adsorption behavior of monovalent cations Li+, Na+, and Cs+ in LiCl, NaCl, and CsCl solutions, respectively, on montmorillonite and quartz surfaces were investigated to reveal adsorption mechanisms at the subatomic scale. The results showed that on the permanently charged montmorillonite surface, both strong-force zero-order and weak-force first-order kinetics were present, whereas only weak-force first-order kinetics were observed on the variably charged quartz surface. Based on the recently established asymmetric orbital hybridization theory to describe cation adsorption at charged surfaces, the origins of the adsorption forces of cations on the two differently charged surfaces were evaluated and quantitatively distinguished. It was concluded that the different forces and their strengths resulted in the differences between the cation adsorption mechanisms at montmorillonite and quartz surfaces. For each type of cation, the total adsorption energy at the montmorillonite surface was much larger than that at the quartz surface; the former was up to three times larger than the latter, depending on the cation concentration. The main forces for cation adsorption at the montmorillonite surface were the electrostatic force and force from asymmetric orbital hybridization, depending on cation type. The main force for cation adsorption at the quartz surface was electrostatic force at a cation concentration of 0.001 mol/L, whereas at a cation concentration of 0.01 mol/L, the electrostatic force approached zero and adsorption was driven by the dispersion force.